Accelerometers are widely used for motion sensing applications. Conventionally, an accelerometer consists of a suspended proof mass and a means of measuring the proof mass displacement with respect to the reference frame. Recent advances in microelectronics technology enabled fabrication of accelerometers with integrated electronics in volume production. One of the first applications of these integrated micromachined sensors was in airbag deployment for automobiles (Analog Device's XL50).
The first accelerometer products that were fabricated using MEMS technology were introduced by large corporations such as Analog Devices, ST and Bosch who had already infrastructure to produce integrated circuits. Integrated circuit fabrication mostly involves depositing several dielectric layers and selectively etching these layers. Therefore the first MEMS accelerometers were fabricated using the same techniques due to the ease of integration with electronics and compatibility with existing CMOS manufacturing equipment.
Building mechanical structures on silicon wafer based on the deposition and etching of different structural layers is called surface micromachining. In surface micromachining usually a sacrificial layer is deposited on a substrate followed by a deposition of mechanical layer where the moving parts of the accelerometer are going to be defined. The moving parts are later released by selectively removing the sacrificial layer. This method has many shortcomings for building low cost and high performance accelerometers. For example, there are contradicting requirements over the area (cost) of the accelerometer and the noise performance. The Brownian noise level of the accelerometer is proportional to the size of the proof mass. In surface micromachining, the proof mass height is determined by the deposited film thickness which is usually limited to less than 10 microns. Therefore, building heavy proof masses requires relatively large area which in return increases the cost.
Surface micromachining also necessitates complex fabrication steps. Depositing thick films which are required for low accelerometer noise is a very sophisticated process. Moreover, non-uniformity of the deposited films and large variation of the material properties have negative impact on the process yield and cost. Controlling stress level in the film is another issue which needs to be dealt with otherwise undesired curling of the released structures may occur. In addition, moveable parts released by using sacrificial wet etching may suffer from the stiction problem if their mechanical properties are not selected properly. Stiction can be avoided by fabricating structures with high spring constants. But this adversely affects the sensitivity of the accelerometer where the sensitivity is inversely proportional to the resonant frequency. Therefore, stiction problem limits the accelerometer sensitivity.
In addition to above described technical difficulties, surface micromachining tools are not readily available to small companies. Most of the required equipment can only be supported by a complicated infrastructure that only large companies can afford. This sets a very high barrier for small start-up companies that want to enter the accelerometer market. Surface micromachining is not a feasible solution for companies which do not have access to the expensive fabrication equipment.
Bulk micromachining, on the other hand, overcomes most of the technical difficulties of surface micromachining as well as it provides a viable solution for fabless semiconductor MEMS companies. In contrast to surface micromachining, bulk micromachining defines structures by selectively etching the substrate. Since the height of the structures is defined in the substrate, it is possible to build accelerometers with increased height and reduced foot print without the complexities associated with building structures using deposited layers. Increased mass in a small foot print provides fabricating accelerometer with better noise performance at a reduced cost. In addition, bulk micromachining techniques are readily available through MEMS foundaries. Bulk micromachined devices can easily be built on off the shelf SOI (silicon on insulator) substrates.
Another important process step for fabricating low cost MEMS device is the integration of mechanical parts with the electronics. To address this need “Nasiri-Fabrication” platform was introduced previously (U.S. Pat. No. 7,104,129, entitled “Vertically integrated MEMS structure with electronics in a hermetically sealed cavity”). This fabrication process makes use of bulk micromachining and readily allows for the wafer level integration of the MEMS substrate and the electronics (CMOS) substrate. In addition to integration, this method encapsulates the mechanical parts in a low pressure hermetically sealed chamber that protects the MEMS device against adverse effect of environment such as humidity. In summary, use of bulk micromachining and water scale MEM-CMOS integration result in low cost and high performance accelerometers. This patent describes a novel accelerometer design that uses bulk silicon machining and Nasiri-Fabrication integration solution.
There is a need for a small low cost high performance accelerometer. The present invention addresses such a need.